Silicon nitride films are used as insulating films, protection films, and so forth in various semiconductor devices. It is known to form a silicon nitride film of this kind by use of a plasma CVD method, which employs a silicon-containing compound gas serving as a source gas, such as silane (SiH4), and a nitrogen-containing compound gas, such as nitrogen or ammonia, (for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-260767).
Where a silicon nitride films is formed by a conventional plasma CVD method, an important study subject is suppression of film stress that causes ill effects on device characteristics, i.e., tensile stress and compressive stress. For example, it is known that, where a silicon nitride film has a large compressive stress, stress migration occurs and causes disconnection of a metal interconnection line directly below the film due to the stress. Accordingly, such a compressive stress should be decreased to prevent this problem. In the case of plasma CVD methods, the direction of stress in a silicon nitride film (whether it is tensile stress or compressive stress) and the degree thereof are determined on the basis of film formation conditions, such as pressure, temperature, and film formation gas type. Conventional, where a silicon nitride film is formed by a plasma CVD method, the film formation conditions are set to prevent a strong stress from being generated in the film (for example, Kazuo Maeda, “VLSI and CVD”, Maki Shoten (publisher), Jul. 31, 1997).
In recent years, attempts have been made for some sorts of devices to improve device characteristics by positively utilizing stress in silicon nitride films. However, for example, plasma CVD apparatuses of the parallel-plate type and of the inductive coupling type use plasma having a relatively high electron temperature. In this case, if a process condition, such as the RF (radio frequency) output, pressure, or temperature, is changed to introduce a high stress, the film formation conditions fall in a state that causes plasma damage to a silicon nitride film formed by the process. This makes it difficult to form a silicon nitride film of high quality in apparatuses of these types. Accordingly, formation of a film with a high stress is difficult. Further, due to a narrow range of selection of plasma process conditions, control of the stress with high accuracy is also difficult.